Aqueous phase catalytic and electrocatalytic hydrogenation of phenol and benzaldehyde over platinum group metals

Abstract

The mechanisms of aqueous-phase thermal catalytic hydrogenation (TCH) and electrocatalytic hydrogenation (ECH) of organic molecules over Pt group metals are not as well-understood as gas-phase thermal catalytic hydrogenation. In gas-phase, the reactions generally occur via a Langmuir-Hinshelwood mechanism with adsorbed hydrogen adding to adsorbed organics. Here, we show that the rates, reaction orders and activation energies for TCH and ECH of phenol and benzaldehyde on Pd, Pt, and Rh can be explained with a simple kinetic model based on similar Langmuir-Hinshelwood mechanisms. For Pt/C, the adsorption equilibrium constants for the organics needed to fit the rate data are consistent with independently-measured values, provided we assume that the rates are dominated by (1 1 1)-like sites, in agreement with reported particle size effects. The reaction rate of Pd in the ECH of benzaldehyde increases with the surface hydrogen coverage. The state of Pd during ECH of phenol and benzaldehyde are very different, with a high concentration of adsorbed H in the presence of phenol, but not in the presence of benzaldehyde, consistent with benzaldehyde’s stronger binding to the surface. In consequence, Pd is converted to β-PdHx during the hydrogenation of phenol but not benzaldehyde. This is proposed to explain the much lower activity of Pd for hydrogenation of phenol compared to benzaldehyde. The measured low coverage of H on Pd in the presence of benzaldehyde is in agreement with the high selectivity/Faradaic efficiency of protons to benzaldehyde hydrogenation to benzyl alcohol. The decrease in apparent activation energy for ECH versus TCH can also be understood within this same kinetic model. The combination of ECH and TCH kinetics and spectroscopy has, thus, allowed to deduce a microkinetic model for these hydrogenation reactions.